Silicon thin film, group of silicon single crystal grains and formation process thereof, and semiconductor device, flash memory cell and fabrication process thereof

ABSTRACT

A process of forming a silicon thin film includes the steps of: irradiating a pulsed rectangular ultraviolet beam on an amorphous or polycrystalline silicon layer formed on a base body, to thereby form a silicon thin film composed of a group of silicon single crystal grains which are each approximately rectangular-shaped and which are arranged in a grid pattern on the base body. In this process, the moved amount of a ultraviolet beam irradiating position in a period from completion of an irradiation of the rectangular ultraviolet beam to starting of the next irradiation of the rectangular ultraviolet beam is specified at 40 μm or less, and a ratio of the moved amount to a width of the rectangular ultraviolet beam measured in the movement direction thereof is in a range of 0.1 to 5%. Further, a selected orientation of the silicon single crystal grains to the surface of the base body is approximately the &lt;100&gt; direction.

BACKGROUND OF THE INVENTION

The present invention relates to a new silicon thin film, a group ofsilicon single crystal grains, and formation processes thereof, and asemiconductor device, a flash memory cell, and fabrication processesthereof.

A silicon thin film composed of a group of silicon single crystal grainsformed on a base body has been used for various kinds of semiconductordevices such as a thin film transistor (hereinafter, referred to as a“TFT”) and a semiconductor device based on a SOI (Silicon On Insulator)technique, and for solar cells; and it is being examined to be appliedto production of micromachines.

In the field of semiconductor devices, for example, a stacked SRAM usingTFTs as load elements has been proposed. TFTs have been also used forLCD (Liquid Crystal Display) panels. And, in general, a silicon thinfilm composed of a group of silicon single crystal grains is used for aTFT required to be enhanced in electric characteristics such as acarrier mobility (μ), conductivity (σ), on-current characteristic,subthreshold characteristic, and on/off current ratio. Concretely,efforts are being made to improve characteristics of SRAMs and TFTs byincreasing sizes of silicon single crystal grains in combination withreduction in density of twin crystals for lowering a trap density in thesilicon single crystal grains.

The enlargement in size (up to 1 μm) of silicon single crystal grainsfor improving the electric characteristics of such a silicon thin filmhas been examined by a SPC [Solid Phase Crystallization, solid phasecrystallization from amorphous silicon) technique or an ELA (ExcimerLaser Anneal, liquid phase crystallization using excimer laser). Oneprocess of forming a silicon thin film using the ELA technique has beenknown, for example, from a document [“Dependence of CrystallizationBehaviors of Excimer Laser Annealed Amorphous Silicon Film on the Numberof Laser Shot”, B. Jung, et al., AM-LCD 95, PP. 177-120]. This documentdescribes that a silicon thin film composed of silicon single crystalgrains whose selected orientation is approximately the <111> directioncan be formed by repeatedly irradiating excimer laser beams on anamorphous silicon layer. Another process of forming a silicon thin filmusing the ELA technique has been also known, for example, from adocument [“Crystal forms by solid-state recrystallization of amorphousSi film on SiO₂”, T. Noma, Appl. Phys. Lett. 59 (6), Aug. 5, 1991, pp.653-655]. This document describes that silicon single crystal grains areoriented in the <110> direction, and they include fine {111} twincrystals.

A process of forming a silicon thin film by graphoepitaxial growth usinga strip-heater has been known, for example, from a document [Silicongraphoepitaxy using a strip-heater oven”, M. W. Geis, et al., Appl.Phys. Lett. 37(5), Sep. 1, 1980, pp. 454-456]. This document describesthat a silicon thin film formed on SiO₂ is composed of a (100)aggregation structure.

A silicon thin film composed of a group of silicon single crystal grainshas been also formed by a chemical-vapor deposition (CVD) process or arandom solid-phase growth process. For example, the formation ofpolysilicon crystal grains by CVD has been known from Japanese PatentLaid-open Nos. Sho 63-307431 and Sho 63-307776. In the techniquesdisclosed in these documents, the selected orientation of silicon singlecrystal grains is the <111> direction. Incidentally, in the case where asilicon thin film composed of a group of silicon single crystal grainshaving large sizes is formed by a normal chemical vapor depositionprocess, it cannot satisfy a uniform quality, a reduced leak, and a highmobility. In the random solid-phase growth process, it is possible toform a silicon thin film composed of a group of silicon single crystalgrains having an average grain size of 1 μm or more; however, it isdifficult for silicon single crystal grains to selectively grow.Further, in the TFT using the silicon thin film formed by such aprocess, since grain boundaries tend to be present in a TFT activeregion, there occurs a problem that TFT characteristics are varieddepending on the presence of the grain boundaries, to thereby shortenthe life time of the TFT.

In all of the techniques disclosed in the above-described references, noattempt has been made to regularly arrange a group of silicon singlecrystal grains on an insulating film. If a group of silicon singlecrystal grains can be regularly arranged on an insulating film, the TFTcharacteristics can be highly controlled and equalized, and also one TFTcan be formed in each of the silicon single crystal grains. This isexpected to further develop the SOI technique.

A process of arranging silicon nuclei or crystal nuclei at desiredpositions and forming silicon single crystal grains having large sizeson the basis of the silicon nuclei or crystal nuclei has been known, forexample, from Japanese Patent Laid-open Nos. Hei 3-125422, Hei 5-226246,Hei 6-97074, and Hei 6-302512. In the technique disclosed in JapanesePatent Laid-open No. Hei 3-125422, micro-sized silicon nuclei or crystalnuclei must be formed by patterning using a lithography process;however, there is a limitation to accurately form these micro-sizedsilicon nuclei or crystal nuclei by the present lithography technique.In the case where the sizes of silicon nuclei or crystal nuclei arelarge, polycrystals tend to be formed with twin crystals anddislocations easily produced, resulting in the reduced throughput. Inthe techniques disclosed in Japanese Patent Lid-open Nos. Hei 5-226246,Hei 6-97074, and Hei 6-302512, it is necessary to irradiate an energybeam enabling fine convergence and direct scanning onto an amorphoussilicon layer or to carry out ion implantation. Accordingly, thesetechniques have problems that not only the step of forming siliconsingle crystal grains is complicated, but also it takes a lot of time toform silicon single crystal grains because of the necessity of a solidphase growth step, resulting in the reduced throughput.

On the other hand, non-volatile memories are being extensively developedat present. In particular, a flash memory having a floating gatestructure is being examined from the viewpoint of the reduced size ofthe memory cell and the lowered voltage. In a flash memory, data iswritten or erased by injecting or discharging an electric charge into orfrom the floating gate. Of various electric charge injecting methods, achannel hot electron injection method or a method of allowing aFowler-Nordheim's tunnel current to flow by applying a high electricfield (for example, 8 MV/cm or more) on a tunnel oxide film aregenerally used.

In such a flash memory cell, it has been known that a threshold voltageafter erasion of data is varied depending on variations in sizes ofpolycrystalline silicon grains forming a floating gate, for example,from a document [“Non-volatile Memory and Its Scaling”, Journal of JapanSociety of Electron Information Communication, Vol. 9, No. 5, pp.469-484 (May, 1996)]. Further, as one means for realizing a future fineflash memory cell operated at a low voltage, a flash memory including afloating gate composed of silicon nanocrystals has been proposed in adocument [“A silicon nanocrystal based memory”, S. Tiwari, et al., Appl.Phys. Lett. 68 (10), 4, pp. 1377-1379, Mar. 4, 1996]. Additionally, asone form of a non-volatile memory to lead the next generation over thepresent semiconductor device, a single electron memory operated at a lowvoltage using a small storage electric charge (electron) has beenproposed in a document [“A Room-temperature Single-Electron MemoryDevice Using Fine-Grain Polycrystalline Silicon”, K. Yano, et. al.,IEDM93, PP. 541-544].

To realize a flash memory cell hard to be varied in a threshold voltageafter erasion of data, it is necessary to make as small as possiblevariations in sizes of silicon crystal grains forming a floating gate.Also, to realize a fine flash memory cell operated at a low voltage, itis necessary to regularly form fine silicon crystal grains on a thininsulating film (tunnel oxide film) with an excellent controllability.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process of easily,effectively forming a group of silicon single crystal grains in a gridpattern on a base body with the reduced variations in grain size, and agroup of silicon single crystal grains formed by the process.

Another object of the present invention is to provide a process ofeasily, effectively forming a silicon thin film composed of a group ofsilicon single crystal grains which are arranged in a grid pattern on abase body with the reduced variations in grain size, and a silicon thinfilm formed by the process.

A further object of the present invention is to provide a process offabricating a semiconductor device using the above silicon thin film orthe above group of silicon single crystal grains, and a semiconductordevice fabricated by the process.

An additional object of the present invention is to provide a process offabricating a flash memory cell using the above silicon thin film or theabove group of silicon single crystal grains, and a flash memory cellfabricated by the process.

To achieve the above object, according to the present invention, thereis provided a process of forming a silicon thin film, including the stepof: irradiating a pulsed rectangular ultraviolet beam on an amorphous orpolycrystalline silicon layer formed on a base body, to thereby form asilicon thin film composed of a group of silicon single crystal grainson the base body; wherein the moved amount or amount of movement (L) ofa ultraviolet beam irradiating position in a period from completion ofan irradiation of the rectangular ultraviolet beam to starting of thenext irradiation of the rectangular ultraviolet beam is specified at 40μm or less, and a ratio (R=L/W) of the moved amount to a width (W) ofthe rectangular ultraviolet beam measured in the movement directionthereof is in a range of 0.1 to 5%, preferably, in a range of 0.5 to2.5%, whereby forming a silicon thin film composed of a group of siliconsingle crystal grains which are each approximately rectangular-shapedand which are arranged in a grid pattern on the base body, a selectedorientation of the silicon single crystal grains to the surface of thebase body being approximately the <100> direction.

According to the present invention, there is provided a silicon thinfilm including a group of silicon single crystal grains which are eachapproximately rectangular-shaped and which are arranged in a gridpattern on a base body, wherein a selected orientation of the siliconsingle crystal grains to the surface of the base body is approximatelythe <100> direction.

To achieve the above object, according to the present invention, thereis provided a process of fabricating a semiconductor device, includingthe steps of: irradiating a pulsed rectangular ultraviolet beam on anamorphous or polycrystalline silicon layer formed on a base body, toform a silicon thin film composed of a group of silicon single crystalgrains on the base body; and forming a source/drain region and a channelregion in the silicon thin film or the silicon single crystal grains;wherein the moved amount (L) of a ultraviolet beam irradiating positionin a period from completion of an irradiation of the rectangularultraviolet beam to starting of the next irradiation of the rectangularultraviolet beam is specified at 40 μm or less, and a ratio (R=L/W) ofthe moved amount to a width (W) of the rectangular ultraviolet beammeasured in the movement direction thereof is in a range of 0.1 to 5%,preferably, in a range of 0.5 to 2.5%, whereby forming a silicon thinfilm composed of a group of silicon single crystal grains which are eachapproximately rectangular-shaped and which are arranged in a gridpattern on the base body, a selected orientation of the silicon singlecrystal grains to the surface of the base body being approximately the<100> direction.

According to the present invention, there is provided a semiconductordevice including a source/drain region and a channel region formed in asilicon thin film composed of a group of silicon single crystal grainswhich are each approximately rectangular-shaped and which are arrangedin a grid pattern on a base body or formed in the silicon single crystalgrains, wherein a selected orientation of the silicon single crystalgrains to the surface of the base body is approximately the <100>direction.

As the semiconductor device of the present invention or thesemiconductor device fabricated by the process of fabricating asemiconductor device of the present invention, there may be exemplifieda top gate type or a bottom gate type thin film transistor used for LCDpanels, a semiconductor device based on the SOI technique (for example,a thin film transistor as a load element of a stacked SRAM), and a MOStype semiconductor device. The silicon thin film and the formationprocess thereof according to the present invention can be applied notonly to fabrication of these semiconductor devices but also toproduction of solar cells and to fabrication of micromachines.

In the silicon thin film and the formation process thereof, and thesemiconductor device and the fabrication process thereof according tothe present invention, a length of one side of a silicon single crystalgrain approximately rectangular-shaped may be 0.05 μm or more,preferably, 0.1 μm or more. Here, the wording “a silicon single crystalgrain approximately rectangular-shaped” means not only a perfectlyrectangular silicon single crystal grain but also an imperfectlyrectangular silicon single crystal grain having a chipped corner. Thelength of one side of an imperfectly rectangular silicon single crystalgrains having a chipped corner means the length of one side of a virtualrectangular single crystal grain obtained by filling up the chippedcorner. The same shall apply hereinafter. The average thickness of asilicon thin film may be in a range of 1×10⁻⁸ m to 1×10⁻⁷ m, preferably,in a range of 1×10⁻⁸ m to 6×10⁻⁸ m, more preferably, in a range of1×10⁻⁸ m to 4×10⁻⁸ m. When the average thickness of a silicon thin filmis less than 1×10⁻⁸ m, there is a difficulty in fabrication of asemiconductor device using the silicon thin film. On the other hand,when it is more than 1×10⁻⁷ m, the thickness of an amorphous orpolycrystalline silicon layer required to ensure the thickness of thesilicon thin film is excessively thick, and consequently, the selectedorientation of the silicon single crystal grains is possibly out ofapproximately the <100> direction. The average thickness of a siliconthin film may be measured by an ellipsometer, interference spectrometer,or the like.

In the silicon thin film and the semiconductor device using the siliconthin film according to the present invention, a group of silicon singlecrystal grains constituting the silicon thin film may be formed byirradiating a pulsed rectangular ultraviolet beam on an amorphous orpolycrystalline silicon layer formed on a base body, and the movedamount (L) of a ultraviolet beam irradiating position in a period fromcompletion of an irradiation of the rectangular ultraviolet beam tostarting of the next irradiation of the rectangular ultraviolet beam maybe specified at 40 μm or less, and a ratio (R=L/W) of the moved amountto a width (W) of the rectangular ultraviolet beam measured in themovement direction thereof may be in a range of 0.1 to 5%, preferably,in a range of 0.5 to 2.5%. In the silicon thin film and the formationprocess thereof, and the semiconductor device and the fabricationprocess thereof according to the present invention, opposed sides of thesilicon single crystal grain approximately rectangular-shaped may beapproximately in parallel to the movement direction of the ultravioletbeam irradiating position or intersect the movement direction of theultraviolet beam irradiating position at approximately 45°. Crystalplanes constituting these two sides are the {220} planes. That is, acrystal plane constituting one side of a silicon single crystal grainapproximately rectangular-shaped is the {220} plane.

To achieve the above object, according to the present invention, thereis provided a process of forming a group of silicon single crystalgrains including: a step (a) of irradiating a pulsed rectangularultraviolet beam on an amorphous or polycrystalline silicon layer formedon a base body, to thereby form a silicon thin film composed of a groupof silicon single crystal grains which are each approximatelyrectangular-shaped and which are arranged in a grid pattern on the basebody, a selected orientation of the silicon single crystal grains to thesurface of the base body being approximately the <100> direction; and astep (b) of separating adjacent ones of the silicon single crystalgrains to each other; wherein the moved amount (L) of a ultraviolet beamirradiating position in a period from completion of an irradiation ofthe rectangular ultraviolet beam to starting of the next irradiation ofthe rectangular ultraviolet beam is specified at 40 μm or less, and aratio (R=L/W) of the moved amount to a width (W) of the rectangularultraviolet beam measured in the movement direction thereof is in arange of 0.1 to 5%, preferably, 0.5 to 2.5%.

According to the present invention, there is provided a group of siliconsingle crystal grains, including a plurality of silicon single crystalgrains which are each approximately rectangular-shaped and which arearranged in a grid pattern on a base body, wherein a selectedorientation of the silicon single crystal grains to the surface of thebase body is approximately the <100> direction, and adjacent ones of thesilicon single crystal grains are separated from each other.

To achieve the above object, according to the present invention, thereis provided a process of fabricating a flash memory cell, including: astep (a) of irradiating a pulsed rectangular ultraviolet beam on anamorphous or polycrystalline silicon layer formed on a tunnel oxidefilm, to form a silicon thin film composed of a group of silicon singlecrystal grains which are each approximately rectangular-shaped and whichare arranged in a grid pattern on the tunnel oxide film, a selectedorientation of the silicon single crystal grains to the surface of thetunnel oxide film is approximately the <100> direction; and a step (b)of separating adjacent ones of the silicon single crystal grains to eachother, whereby forming a floating gate composed of the group of siliconsingle crystal grains; wherein the moved amount (L) of a ultravioletbeam irradiating position in a period from completion of an irradiationof the rectangular ultraviolet beam to starting of the next irradiationof the rectangular ultraviolet beam is specified at 40 μm or less, and aratio (R=L/W) of the moved amount to a width (W) of the rectangularultraviolet beam measured in the movement direction thereof is in arange of 0.1 to 5%, preferably, 0.5 to 2.5%.

According to the present invention, there is provided a flash memorycell including a floating gate composed of a plurality of silicon singlecrystal grains which are each approximately rectangular-shaped and whichare formed on a tunnel oxide film, a selected orientation of the siliconsingle crystal grains to the surface of the tunnel oxide film beingapproximately the <100> direction; wherein the silicon single crystalgrains are arranged in a grid pattern on the tunnel oxide film andadjacent ones of the silicon single crystal grains are separated fromeach other. In addition, the thickness of each of the silicon singlecrystal grains separated from each other may be in a range of 1×10⁻⁸ mto 1×10⁻⁷ m, preferably, 1×10⁻⁸ m to 8×10⁻⁸ m, more preferably, in arange of 2×10⁻⁸ m to 5×10⁻⁸ m.

The flash memory cell of the present invention or the flash memory cellfabricated by the process of fabricating a flash memory cell of thepresent invention basically includes a source/drain region and a channelregion formed in a semiconducting substrate or a silicon layer, a tunneloxide film formed thereon, a floating gate formed on the tunnel oxidefilm, an insulating film covering the floating gate, and a control gate.

In the group of silicon single crystal grains or the flash memory cellusing the same according to the present invention, the group of siliconsingle crystal grains may be formed by a process including: a step (a)of irradiating a pulsed rectangular ultraviolet beam on an amorphous orpolycrystalline silicon layer formed on the base body (or tunnel oxidefilm), to thereby form a silicon thin film composed of a group ofsilicon single crystal grains which are each approximatelyrectangular-shaped and which are arranged in a grid pattern on the basebody (tunnel oxide film), a selected orientation of the silicon singlecrystal grains to the surface of the base body (tunnel oxide film) beingapproximately the <100> direction; and a step (b) of separating adjacentones of the silicon single crystal grains to each other; wherein themoved amount (L) of a ultraviolet beam irradiating position in a periodfrom completion of an irradiation of the rectangular ultraviolet beam tostarting of the next irradiation of the rectangular ultraviolet beam isspecified at 40 μm or less, and a ratio (R=L/W) of the moved amount to awidth (W) of the rectangular ultraviolet beam measured in the movementdirection thereof is in a range of 0.1 to 5%, preferably, in a range of0.5 to 2.5%.

In the group of silicon single crystal grains and the formation processthereof, and the flash memory cell and the fabrication process thereofaccording to the present invention, the step (b) of separating adjacentones of the silicon single crystal grains to each other preferablyincludes a step of oxidizing the silicon thin film formed in the step(a) to form each region made of silicon oxide between the adjacent onesof the silicon single crystal grains. Alternatively, the step (b) ofseparating adjacent ones of the silicon single crystal grains to eachother preferably includes a step of etching the silicon thin film formedin the step (a) to form each space between the adjacent ones of thesilicon single crystal grains. The length of one side of each of theapproximately rectangular-shaped silicon single crystal grains in thesilicon thin film formed in the step (a) is preferably as short aspossibly; however, it is preferably 0.05 μm or more from a practicalstandpoint. The average thickness of the silicon thin film formed in thestep (a) may be in a range of 1×10⁻⁸ m to 1×10⁻⁷ m, preferably, 1×10⁻⁸ mto 6×10⁻⁸ m, more preferably, 1×10⁻⁸ m to 4×10⁻⁸ m. The opposed sides ofeach of the approximately rectangular-shaped silicon single crystalgrains in the silicon thin film formed in the step (a) may beapproximately in parallel to the movement direction of the ultravioletbeam irradiating position or intersect the movement direction of theultraviolet beam irradiating position at approximately 45°. Crystalplanes constituting these two sides are the {220} planes. That is, acrystal plane constituting one side of a silicon single crystal grainapproximately rectangular-shaped is the {220} plane.

As the base body or the tunnel oxide film in the present invention,there may be exemplified, while not exclusively, silicon oxide (SiO₂),silicon nitride (SiN), SiON, a stacked structure of SiO₂—SiN, and astacked structure of SiO₂—SiN—SiO₂. The base body or the tunnel oxidefilm may be formed by oxidization or nitrization of the surface of asilicon semiconducting substrate, or may be formed of a suitable film ona semiconducting substrate, a layer, or an interconnection layer by CVDor the like.

As a ultraviolet beam, there may be exemplified a XeCl excimer laserhaving a wavelength of 308 nm and a full-solid ultraviolet laser. Thewidth (W) of a rectangular ultraviolet beam measured in the movementdirection is preferably in a range of 40 μm to about 1 mm. The length ofa rectangular ultraviolet beam measured in the direction perpendicularto the movement direction may be freely selected. It is desired to use aultraviolet beam having an extremely sharp rise in energy at an edgeportion. As a ultraviolet beam source for generating such a ultravioletbeam, there may be exemplified, while not exclusively, a combination ofa XeCl excimer laser source, and attenuator, a beam homogenizer forequalizing a rectangular beam, and a reflection mirror.

When the moved amount (L) of a ultraviolet beam irradiating position ina period from completion of an irradiation of a rectangular ultravioletbeam to stating of the next irradiation of the rectangular ultravioletbeam is more than 40 μm or a ratio (R=L/W) of the moved amount to thewidth (W) of the ultraviolet beam measured in the movement direction ofthe ultraviolet beam irradiating position is more than 5%, there is afear that a group of silicon single crystal grains which are eachapproximately rectangular shaped and which are arranged in a gridpattern on a base body are not formed, or that the selected orientationof the silicon single crystal grains to the surface of the base body isout of approximately the <100> direction. Further, when the movementratio (R=L/W) is less than 0.1% of the width (W) of the ultraviolet beammeasured in the movement direction of the ultraviolet beam irradiatingposition, the throughput becomes excessively low. In addition, the basebody may be moved with the ultraviolet beam source kept fixed; theultraviolet beam source may be moved with the base body kept fixed; orboth the ultraviolet beam source and the base body may be moved.

In the case where 30% or more of silicon single crystal grainsconstituting a group of silicon single crystal grains are selectivelyoriented approximately in the <100> direction with respect to thesurface of a base body, the selected orientation of the silicon singlecrystal grains constituting the group of silicon single crystal grainsto the surface of a base body is specified as approximately the <100>direction. In addition, the approximately <100> direction of siliconsingle crystal grains contains the case that the <100> direction of thesilicon single crystal grains is not strictly in parallel to thedirection perpendicular to the surface of a base body. The selectedorientation is sometimes called “a preferred orientation”. Apolycrystalline structure in the form of a film or the like in whichcrystals are not random-oriented but a large number of the crystals havea crystal axis, crystal plane and the like oriented in a specifieddirection, is called “an aggregate structure” or “a fiber structure”. Inthis structure, the oriented crystal axis is called the selectedorientation.

In the present invention, as described above, by specifying the movedamount (L) of a ultraviolet beam irradiating position in a period fromcompletion of an irradiation of a rectangular ultraviolet beam tostarting of the next irradiation of the rectangular ultraviolet beam tobe 40 μm or less, and also specifying a ratio (R=L/W) of the movedamount to the width (W) of the ultraviolet beam measured in the movementdirection of the ultraviolet beam irradiating position to be in a rangeof 0.1 to 5%, there can be formed a silicon thin film composed of agroup of silicon single crystal grains which are each approximatelyrectangular-shaped and which are arranged in a grid pattern on a basebody, a selected orientation of the silicon single crystal grains beingapproximately the <100> direction. The reason for this is unclear, butit may be considered as follows: namely, by irradiating a pulsedrectangular ultraviolet beam (having extremely sharp rise in energy atan edge portion) in a certain region of an amorphous or polycrystallinesilicon layer while overlapping and slightly shifting the ultravioletbeams, there is established a repetition of a thermal equilibrium stateby heat reservation and a cooling (solidifying) state, to thereby form agroup of these silicon single crystal grains. Also, the reason why theselected orientation of silicon single crystal grains to the surface ofa base body is approximately the <100> direction may be considered to bedue to the free energy on the surface of Si to the base body made of,for example, SiO₂.

According to the present invention, there can be easily, effectivelyformed a silicon thin film composed of a group of silicon single crystalgrains arranged in a grid pattern on a base body (insulating film), aselected orientation of the silicon single crystal grains to the surfaceof the base body being approximately the <100> direction. Accordingly,it becomes possible to highly control and equalize characteristics of aTFT formed of the silicon thin film or to improve a semiconductor devicebased on the SOI technique by forming a TFT in a fine silicon singlecrystal grain. Further, since the crystallinity of the silicon thin filmis improved in terms of macro-structure, characteristics of a TFT usedfor LCD panels and the like can be enhanced. Also, there can be realizeda flash memory cell (nano dot memory) capable of being operated at a lowvoltage by directly applying a tunneling effect and electronaccumulation. Additionally, by forming a floating gate of a flash memorycell of a silicon thin film of the present invention, variations insizes of silicon grains forming the floating gate can be reduced, sothat there can be realized a flash memory cell hard to be varied inthreshold voltage after erasion of data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view illustrating a process of forming a siliconthin film in Example 1, and

FIGS. 1B and 1C are schematic sectional views of the silicon thin filmformed on a base body;

FIG. 2 is a transmission type electron microscopic photograph of thesilicon thin film obtained in Example 1;

FIG. 3 is a copy of an AFM photograph of the surface of the silicon thinfilm obtained in Example 1;

FIG. 4 is a copy of another AFM photograph of the surface of the siliconthin film obtained in Example 1;

FIG. 5 is a copy of an AFM photograph of the surface of a silicon thinfilm obtained in Comparative Example 1;

FIG. 6 is a copy of an AFM photograph of the surface of a silicon thinfilm obtained in Comparative Example 2;

FIGS. 7A to 7C are schematic sectional views of a silicon thin filmformed on a base body, illustrating a process of fabricating asemiconductor device in Example 2;

FIG. 8 is a schematic sectional view of a bottom gate type thin filmtransistor in Example 2;

FIG. 9 is a graph showing evaluation results of characteristics ofn-type thin film transistors of a bottom gate type in Examples 2, 3 andComparative Example 2;

FIGS. 10A to 10C are schematic sectional views of a silicon thin filmformed on a tunnel oxide film, illustrating a process of fabricating aflash memory cell in Example 4; and

FIGS. 11A and 11B are schematic sectional views, continued to FIGS. 10Ato 10C, illustrating the process of fabricating a flash memory cell inExample 4; and

FIG. 12 is a schematic sectional view showing one example in which afloating gate of a flash memory cell is formed of the silicon thin filmof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in detail using thefollowing examples with reference to the accompanying drawings.

EXAMPLE 1

This example concerns a silicon thin film and a formation processthereof according to the present invention. In this example, a siliconthin film composed of a group of silicon single crystal grains wasformed on a base body made of SiO₂ by irradiating a pulsed ultravioletbeam on an amorphous silicon layer formed on the base body. Theirradiation conditions of the ultraviolet beam and the like are shown inTable 1.

TABLE 1 ultraviolet beam XeCl excimer laser (wavelength: 308 nm)irradiated amount 320 mJ/cm² pulse width about 26 nano seconds frequencyabout 200 Hz beam shape rectangular shape: 400 μm (width, W) × 150 mm(length) moved amount L 4 μm movement ratio R 1% (= 4 μm/400 μm × 100)

Concretely, a SiN film 11 having a thickness of 50 nm was formed on asubstrate 10 made of quartz and a base body 12 made of SiO₂ was formedon the SiN film 11 to a thickness of 100 nm. Then, an amorphous siliconlayer 13 having a thickness of 30 nm was formed on the base body 12 byPECVD. This state is shown by a schematic sectional view of FIG. 1A.Next, a pulsed ultraviolet beam was irradiated on the amorphous siliconlayer 13 formed on the base body 12 in the conditions shown in Table 1.This state is shown by a schematic sectional view of FIG. 1B. In FIG.1B, a region of the silicon layer 13 on which the preceding ultravioletbeam was irradiated is shown by the dotted line, and a region of thesilicon layer 13 on which the present ultraviolet beam was irradiated isshown by the chain line. In this example, since the movement ratio(R=L/W) is 1%, a certain point of the amorphous silicon layer 13 is 100times exposed to the pulsed ultraviolet beam. Although the substrate 10was moved with a ultraviolet beam source kept fixed in this example, theultraviolet beam source may be moved with the substrate 10 kept fixed,or both the substrate 10 and the ultraviolet beam source may be moved. Aschematic sectional view of a silicon thin film 14 thus obtained isshown in FIG. 1C. In FIG. 1C, grain boundaries are shown by the dottedline. Each of the silicon single crystal grains has a sectional shape inwhich the central portion is recessed and the peripheral portionprojects.

The silicon thin film thus obtained was observed by a transmission typeelectron microscope. The result is shown by an electron microscopicphotograph in FIG. 2. In addition, a sample for microscopic observationis only a silicon thin film obtained by etching the substrate 10, SiNfilm 11 and base body 12 using a mixed solution of HF/H₂O=½. As can beseen from the photograph of FIG. 2, the silicon thin film thus obtainedis composed of a group of silicon crystal grains which are approximatelyrectangular-shaped. The selected orientation of the silicon singlecrystal grains to the surface of the base body was approximately the<100> direction. The length of one side of the silicon single crystalgrain approximately rectangular-shaped was 0.1 μm or more. The opposedtwo sides of the silicon single crystal grain approximatelyrectangular-shaped were approximately in parallel to the movementdirection of the ultraviolet beam irradiating position. Crystal planesconstituting the two sides were the {220} planes. Depending on theobservation points, the opposed two sides of the silicon single crystalgrain approximately rectangular-shaped intersected the movementdirection of the ultraviolet beam irradiating position at approximately45°.

When a polycrystalline silicon is perfectly non-oriented, a ratio of arefraction intensity I₁₁₁ at the {111} plane to a refraction intensityI₂₂₀ at the {220} plane was I₁₁₁:I₂₂₀=5:3. Besides, the ratio ofI₁₁₁:I₂₂₀ in the silicon thin film obtained in this example was 1:4.From the analysis of the refraction intensity ratio, it is revealed thatthe selection orientation of the silicon single crystal grains to thesurface of the base body is approximately the <100> direction. Inaddition, about 30% of the silicon single crystal grains constitutingthe group of silicon single crystal grains in the silicon thin film wereselectively orientated in the <100> direction to the surface of the basebody, and the remaining silicon single crystal grains were oriented atrandom to the surface of the base body. Also, it was frequently observedthat the adjacent ones in a unit constituted of several silicon singlecrystal grains correspond to each other in the crystal orientation.

The surface of the silicon thin film obtained in this example wasobserved and measured by an AFM (Atomic Force Microscope). The measuredresults are shown in Table 3, and the surface observation photographsare shown in FIGS. 3 and 4. In addition, the observation field in FIG. 3is 3 μm×3 μm, and the observation field in FIG. 4 is 20 μm×20 μm. As canbe seen from FIGS. 3 and 4, the silicon thin film is composed of a groupof silicon single crystal grains arranged in a grid pattern on the basebody. That is, the silicon single crystal grains are regularly arrangedin a checkerboard pattern. Also, in FIG. 4, there are observed somelinear stripes which leftward, downward extend from the right, upperportion. A gap between the stripe is about 4 μm, which substantiallycorresponds to the moved amount or amount of movement L of theultraviolet beam irradiating position. The opposed two sides of thesilicon single crystal grain approximately rectangular-shaped wereapproximately in parallel to the movement direction of the ultravioletbeam irradiating position or intersected the movement direction of theultraviolet beam irradiating position at approximately 45°.

Comparative Example 1

A silicon thin film was formed on a base body in the same manner as inExample 1, except that the moved amount L and the movement ratio R weredifferent from those in Example 1. The moved amount L and the movementratio R are shown in Table 2.

The surfaces of the silicon thin films obtained in Comparative Example 1were observed and measured. The measured results are shown in Table 3,and the surface observation photographs are shown in FIG. 5 (ComparativeExample 1A) and FIG. 6 (Comparative Example 1B). In addition, theobservation field in each of FIGS. 5 and 6 is 3 μm×3 μm. As can be seenfrom FIGS. 5 and 6, for each of the silicon thin films obtained with themoved amount L set at 40 μm or more, single crystal grains are notarranged in a grid pattern on the base body. Further, it is revealedthat as the moved amount L is made larger, the number of irregularitiesof the silicon thin film is made smaller.

Further, experiments were repeated with the irradiated amount of theXeCl excimer laser beam changed into 280 mJ/cm², 320 mJ/cm², 340 mJ/cm²and 360 mJ/cm²; however, any silicon thin film composed of siliconsingle crystal grains selectively oriented in the <100> direction to thesurface of the base body was not formed.

TABLE 2 moved amount L (μm) movement ratio R (%) Comparative  40  10Example 1A Comparative 200  50 Example 1B Comparative 400 100 Example 1C

TABLE 3 movement ratio R Ra (μm) RMS (μm) Example 1  1% 11.71  14.50Comparative  10% 8.66 10.83 Example 1A Comparative  50% 4.81  5.98Example 1B Comparative 100% 5.21  6.30 Example 1C

EXAMPLE 2

This example concerns a semiconductor device and a fabrication processthereof according to the present invention. In this example, an n-typethin film transistor having a bottom gate structure was fabricated usingthe process of forming a silicon thin film described in Example 1. Aninsulating layer 21 made of SiO₂ was formed on the surface of a glasssubstrate 20, and a polycrystalline silicon layer doped with an impuritywas deposited over the surface by CVD. The polycrystalline silicon layerwas patterned, to form a gate electrode 22. Next, a base body 23 made ofSiO₂ was formed over the surface by CVD. The base body 23 also functionsas a gate oxide film.

As in Example 1, an amorphous silicon layer 24 having a thickness of 40nm was formed on the base body 23 made of SiO₂ by PECVD, as shown inFIG. 7A. Next, a pulsed ultraviolet beam was irradiated on the amorphoussilicon layer 24 thus formed (see FIG. 7B), to form a silicon thin film25 composed of a group of silicon single crystal grains on the base body(see FIG. 7C). The irradiation conditions of the ultraviolet beam andthe like are the same as those shown in Table 1. In addition, a regionof the silicon layer 24 on which the preceding ultraviolet beam wasirradiated is shown by the dotted line, and a region of the siliconlayer 24 on which the present ultraviolet beam was irradiated is shownby the chain line.

An impurity was doped in a source/drain region forming area of thesilicon thin film 25 by ion implantation, followed by activation of theimpurity thus doped, to form a source/drain region 26 and a channelregion 27. An insulating layer 28 made of SiO₂ was then deposited byCVD, and opening portions were formed in the insulating layer 28 atpositions over the source/drain region 26 by lithography and RIE. Aninterconnection material layer made of an aluminum alloy was depositedon the insulating layer 28 including the opening portions by sputtering,followed by patterning of the interconnection material layer, to form aninterconnection 29 on the insulating layer 28 (see FIG. 8). Theinterconnection 29 is connected to the source/drain region 26 via theinterconnection material layer buried in the opening portions.

EXAMPLE 3

An n-type thin film transistor having a bottom gate structure wasfabricated in the same manner as in Example 2, except that the movedamount L and the moved amount/beam width R were different from those inExample 2. The moved amount L and the moved amount/beam width R in eachof Examples 2 and 3 are shown in Table 4.

Comparative Example 2

An n-type thin film transistor having a bottom gate structure wasfabricated in the same manner as in Example 2, except that the movedamount L and the moved amount/beam width R were different from those inExample 2. The moved amount L and the moved amount/beam width R inComparative Examples 2 are shown in Table 4.

The characteristics of the n-type thin film transistors of a bottom gatetype obtained in Examples 2, 3 and Comparative Examples 2A, 2B wereevaluated by measuring a drain current (I_(ON)) with V_(d)=10 V andV_(g)=15 V. The results are shown in FIG. 9. As can be seen from FIG. 9,when the movement R is 5% or less, the drain current (I_(ON)) becomeshigher.

TABLE 4 moved amount L (μm) movement ratio R (%) Example 2  4  1 Example3  20  5 Comparative  40 10 Example 2A Comparative 360 90 Example 2B

EXAMPLE 4

This example concerns a group of silicon single crystal grains and aformation process thereof, and a flash memory cell and a fabricationprocess thereof according to the present invention. Hereinafter, thisexample will be described with reference to FIGS. 10 and 11.

First, element isolation regions 31 having a LOCOS structure was formedin a silicon semiconducting substrate 30, followed by ion implantationfor well formation, channel stop, and threshold value adjustment. Theelement isolation region may be of a trench structure. After that, fineparticles and metal impurities on the surface of the siliconsemiconducting substrate 30 were removed by RCA cleaning, and then thesurface of the silicon semiconducting substrate 30 was cleaned by asolution of 0.1% hydrofluoric acid. Next, a tunnel oxide film(equivalent to a base body) 32 having a thickness of 3 nm was formed onthe exposed surface of the silicon semiconducting substrate 30 by aknown oxidation process.

After that, as shown in FIG. 10A, an amorphous silicon layer 33 having athickness of about 40 nm was formed on the tunnel oxide film 32 byPECVD, as in Example 1. A pulsed ultraviolet beam was irradiated on theamorphous silicon layer 33 (see FIG. 10B), to form a silicon thin film34 composed of a group of silicon single crystal grains on the tunneloxide film 32 (see FIG. 10C). The irradiation conditions of theultraviolet beam and the like are the same as those shown in Table 1. InFIG. 10B, a region of the silicon layer 33 on which the precedingultraviolet beam was irradiated is shown by the dotted line, and aregion of the silicon layer on which the present ultraviolet beam wasirradiated is shown by the chain line.

The silicon thin film thus obtained was observed by a transmission typeelectron microscope and an AFM. It was observed that silicon singlecrystal grains 35 each having an approximately rectangular shape (lengthof one side: about 0.3 μm) were arranged in a grid pattern on the basebody. The selected orientation of the silicon single crystal grains tothe surface of the base body was approximately the <100> direction. Inaddition, about 30% of the silicon single crystal grains constitutingthe group of the silicon single crystal grains were oriented in the<100> direction to the surface of the base body, the remaining siliconsingle crystal grains were random-oriented to the surface of the basebody. Also, in some silicon single crystal grains, the <100> directionwas not strictly in parallel to the direction perpendicular to thesurface of the base body. Further, it was frequently observed that theadjacent ones in a unit constituted of several silicon single crystalgrains correspond to each other in the crystal orientation.Additionally, opposed two sides of the silicon single crystal grainapproximately rectangular-shaped were approximately in parallel to themovement direction of the ultraviolet beam irradiating position orintersected the movement direction of the ultraviolet beam irradiatingposition at approximately 45°.

After that, the adjacent silicon single crystal grains 35 were separatedfrom each other. Concretely, the silicon thin film thus obtained wasoxidized in an oxygen gas atmosphere under the temperature condition of1000° C.×20 min to form each region 36 made of silicon oxide (SiO₂)between the adjacent silicon single crystal grains 35A (see FIG. 11A).The average thickness of the silicon single crystal grain 35A thusoxidized was about 10 nm, and the size thereof was 7-13 nm. Thesesilicon single crystal grains 35A, spaced at intervals (about 0.3 μm),were arranged in a grid pattern on the tunnel oxide film (base body) 32.That is, the silicon single crystal grains were regularly arrange in acheckerboard pattern. Thus, a floating gate 37 composed of a pluralityof the silicon single crystal grains 35A was formed. In general, theoxidation of silicon preferentially proceeds from grain boundaries. Thesilicon single crystal grains 35 was selectively orientatedapproximately in the <100> direction to the surface of the tunnel oxidefilm (base body) 32, so that the thicknesses and the sizes of thesilicon single crystal grains can be preferably adjustable.

After that, the regions 36 made of SiO₂ were patterned, to remove theunnecessary regions 36 made of SiO₂ and the silicon single crystalgrains 35A. Then, an insulating film 38 was formed over the entiresurface by CVD, and a polycrystalline silicon layer doped with animpurity was formed on the insulating film 38 by CVD, followed bypatterning of the polycrystalline silicon layer and the insulating film38. Thus, a control gate 39 formed of the polycrystalline silicon layerwas formed.

After that, an impurity was doped in a source/drain region forming areaof the exposed silicon semiconducting substrate 30 by ion implantation,followed by activation of the impurity thus doped, to form asource/drain region 40 and a channel region 41 (see FIG. 11B). Then, aninsulating film made of, for example SiO₂ was deposited over the entiresurface by CVD, and opening portions were formed in the insulating layerat positions over the source/drain region 40 by photolithography andRIE. An interconnection material layer made of an aluminum alloy wasthen deposited on the insulating layer including the opening portions bysputtering, followed by patterning of the interconnection materiallayer, to accomplish an interconnection on the insulating layer. Theinterconnection is connected to the source/drain region 40 via theinterconnection material layer buried in the opening portions. Thus, aflash memory cell (nano dot memory) was fabricated.

While the present invention has been described with reference to theexamples, such description is for illustrative purposes only, and it isunderstood that many changes may be made without departing from thescope of the present invention. For example, although the amorphoussilicon layer was formed on the base body in the examples, apolycrystalline silicon layer may be formed on the base body. When apulsed ultraviolet beam is irradiated on an amorphous orpolycrystallinbe silicon layer, the base body may be heated. The surfaceof a silicon thin film may be planarized by etching-back. Additionally,in the semiconductor device and the fabrication process thereofaccording to the present invention, one transistor element can befabricated from one silicon single crystal grain formed on a base bodyby forming a source/drain region and a channel region in the siliconsingle crystal grain. In this case, the adjacent silicon single crystalgrains may be separated from each other by patterning a silicon thinfilm by lithography and etching for removing the unnecessary siliconsingle crystal grains. Alternatively, the adjacent silicon singlecrystal grains may be separated from each other by oxidizing a siliconthin film formed on a base body made of a material having an etchingselection ratio to silicon oxide, for example, silicon nitride, to formeach region made of silicon oxide between the adjacent silicon singlecrystal grains, and etching the silicon oxide. Further, as shown by aschematic sectional view of FIG. 12, a floating gate of a flash memorycell may be formed of a silicon thin film according to the presentinvention or a floating gate of a flash memory cell may be formed on thebasis of the process of forming a silicon thin film according to thepresent invention.

What is claimed is:
 1. A process of forming a silicon thin film,comprising the steps of: irradiating a pulsed rectangular ultravioletbeam on an amorphous or polycrystalline silicon layer formed on a basebody, to thereby form a silicon thin film composed of a group of siliconsingle crystal grains on said base body; moving the beam with the movedamount of an ultraviolet beam irradiating position in a period fromcompletion of an irradiation pulse of said rectangular ultraviolet beamto starting of the next irradiation pulse of said rectangularultraviolet beam being in a range of 4 μm to 20 μm, and a ratio of saidmoved amount to a width of said rectangular ultraviolet beam measured inthe movement direction thereof being in a range of 0.1 to 5%, to form asilicon thin film composed of a group of silicon single crystal grainswhich are each approximately rectangular-shaped and which are arrangedin a grid pattern on said base body, a selected orientation of saidsilicon single crystal grains to the surface of said base body beingapproximately the <100> direction.
 2. A process of forming a siliconthin film according to claim 1, wherein a length of one side of saidsilicon single crystal grain approximately rectangular-shaped is 0.05 μmor more.
 3. A process of forming a silicon thin film according to claim1, wherein an average thickness of said silicon thin film is in a rangeof 1×10⁻⁸ m to 1×10⁻⁷ m.
 4. A process of forming a silicon thin filmaccording to claim 1, wherein said base body is made of silicon oxide orsilicon nitride.
 5. A process of forming a silicon thin film accordingto claim 1, wherein opposed sides of said silicon single crystal grainapproximately rectangular-shaped are approximately in parallel to themovement direction of the ultraviolet beam irradiating position orintersect the movement direction of the ultraviolet beam irradiatingposition at approximately 45°.
 6. A process of forming a group ofsilicon single crystal grains comprising: a step (a) of irradiating apulsed rectangular ultraviolet beam on an amorphous or polycrystallinesilicon layer formed on a base body, to thereby form a silicon thin filmcomposed of a group of silicon single crystal grains which are eachapproximately rectangular-shaped and which are arranged in a gridpattern on said base body, a selected orientation of said silicon singlecrystal grains to the surface of said body being approximately the <100>direction; and a step (b) of separating adjacent ones of said siliconsingle crystal grains to each other; wherein the moved amount of anultraviolet beam irradiating position in a period from completion of anirradiation pulse of said rectangular ultraviolet beam to starting ofthe next irradiation pulse of said rectangular ultraviolet beam being ina range of 4 μm to 20 μm, and a ratio of said moved amount to a width ofsaid rectangular ultraviolet beam measured in the movement directionthereof being in a range of 0.1 to 5%.
 7. A process of forming a groupof silicon single crystal grains according to claim 6, wherein said step(b) of separating adjacent ones of said silicon single crystal grainsfrom each other comprises a step of oxidizing said silicon thin filmformed in said step (a) to form a region made of silicon oxide betweenthe adjacent ones of said silicon single crystal grains.
 8. A process offorming a group of silicon single crystal grains according to claim 6,wherein a length of one side of each of said approximatelyrectangular-shaped silicon single crystal grains in said silicon thinfilm formed in said step (a) is 0.05 μm or more.
 9. A process of forminga group of silicon single crystal grains according to claim 6, whereinan average thickness of said silicon thin film formed in said step (a)is in a range of ×10⁻⁸ m to 1×10⁻⁷ m.
 10. A process of forming a groupof silicon single crystal grains according to claim 6, wherein said basebody is made of silicon oxide or silicon nitride.
 11. A process offorming a group of silicon single crystal grains according to claim 6,wherein opposed sides of each of said approximately rectangular-shapedsilicon single crystal grains in said silicon thin film formed in saidstep (a) are approximately in parallel to the movement direction of theultraviolet beam irradiating position or intersect the movementdirection of the ultraviolet beam irradiating position at approximately45°.
 12. A process of fabricating a semiconductor device, comprising thesteps of: irradiating a pulsed rectangular ultraviolet beam on anamorphous or polycrystalline silicon layer formed on a base body, toform a silicon thin film composed of a group of silicon single crystalgrains on said base body; and forming a source/drain region and achannel region in said silicon thin film or said silicon single crystalgrains; wherein the moved amount of an ultraviolet beam irradiatingposition in a period from completion of an irradiation pulse of saidrectangular ultraviolet beam to starting of the next irradiation pulseof said rectangular ultraviolet beam being in a range of 4 μm to 20 μm,and a ratio of said moved amount to a width of said rectangularultraviolet beam measured in the movement direction thereof being in arange of 0.1 to 5%, whereby forming a silicon thin film composed of agroup of silicon single crystal grains which are each approximatelyrectangular-shaped and which are arranged in a grid pattern on said basebody, a selected orientation of said silicon single crystal grains tothe surface of said base body being approximately the <100> direction.13. A process of fabricating a semiconductor device according to claim12, wherein a length of one side of said silicon single crystal grainapproximately rectangular-shaped is 0.05 μm or more.
 14. A process offabricating a semiconductor device according to claim 12, wherein anaverage thickness of said silicon thin film is in a range of 1×10⁻⁸ m to1×10⁻⁷ m.
 15. A process of fabricating a semiconductor device accordingto claim 12, wherein said base body is made of silicon oxide or siliconnitride.
 16. A process of fabricating a semiconductor device accordingto claim 12, wherein opposed sides of said silicon single crystal grainapproximately rectangular-shaped are approximately in parallel to themovement direction of the ultraviolet beam irradiating position orintersect the movement direction of the ultraviolet beam irradiatingposition at approximately 45°.
 17. A process of fabricating a flashmemory cell, comprising: a step (a) of irradiating a pulsed rectangularultraviolet beam on an amorphous or polycrystalline silicon layer formedon a tunnel oxide film, to form a silicon thin film composed of a croupof silicon single crystal grains which are each approximatelyrectangular-shaped and which are arranged in a grid pattern on saidtunnel oxide film, a selected orientation of said silicon single crystalgrains to the surface of said tunnel oxide film is approximately the<100> direction; and a step (b) of separating adjacent ones of saidsilicon single crystal grains to each other, whereby forming a floatinggate composed of said group of silicon single crystal grains; whereinthe moved amount of an ultraviolet beam irradiating position in a periodfrom completion of an irradiation pulse of said rectangular ultravioletbeam to starting of the next irradiation pulse of said rectangularultraviolet beam being in a range of 4 μm to 20 μm, and a ratio of saidmoved amount to a width of said rectangular ultraviolet beam measured inthe movement direction thereof being in a range of 0.1 to 5%.
 18. Aprocess of fabricating a flash memory cell according to claim 17,wherein said step (b) of separating adjacent ones of said silicon singlecrystal grains from each other comprises a step of oxidizing saidsilicon thin film formed in said step (a) to form a region made ofsilicon oxide between the adjacent ones of said silicon single crystalgrains.
 19. A process of fabricating a flash memory cell according toclaim 17, wherein a length of one side of each of said approximatelyrectangular-shaped silicon single crystal grains in said silicon thinfilm formed in said step (a) is 0.05 μm or more.
 20. A process offabricating a flash memory cell according to claim 17, wherein anaverage thickness of said silicon thin film formed in said step (a) isin a range of 1×10⁻⁸ m to 1×10⁻⁷ m.
 21. A process of fabricating a flashmemory cell according to claim 17, wherein opposed sides of each of saidapproximately rectangular-shaped silicon single crystal grains in saidsilicon thin film formed in said steo (a) are approximately in parallelto the movement direction of the ultraviolet beam irradiating positionor intersect the movement direction of the ultraviolet beam irradiatingposition at approximately 45° .